The present invention relates to apparatus for generating a magnetic high-frequency field and/or for receiving corresponding high-frequency signals in a nuclear spin resonance apparatus, and especially such apparatus as used in medical technology having a high-frequency surface coil which can be arranged within an at least partially homogeneous magnetic field region of a magnetic field which is generated by a base field magnet and is oriented along an axis, either directly on a body or part of the body to be examined or at least in the vicinity thereof. Such a high-frequency device is known, for instance, from reports of the "Second Annual Meeting of the Society of Magnetic Resonance in Medicine", Aug. 16 to 19, 1983, San Francisco, U.S.A. See particularly pages 16 and 17, 53 and 54 or 83 and 84.
Especially in the field of medical technology, diagnostic methods have been developed in which integral resonance signals from nuclei of a given element are analyzed by computation or measurement from the spin density distribution and/or from the relaxation time distribution of a body or part of a body to be examined. Such methods are known under the designation "Nuclear Magnetic Resonance Tomography" or "Zeugmatography". A prerequisite for nuclear spin tomography is a strong magnetic field which is generated by a so-called base field magnet, which is as homogeneous as possible in a region of predetermined extent and into which the body or part of a body to be examined is placed along an axis which generally coincides with the orientation axis of the magnetic base field. For exciting the atomic nuclei of a given element in the body into a precession motion, an additional separate coil is required, by which a high-frequency a-c field (RF a-c field) can be generated for a short time and which can also serve for receiving the RF signals associated therewith unless a separate measuring coil is provided. Optionally, the base field can also be varied locally in order to limit the region which is excited by the high-frequency signal of the surface coil (topological NMR; sensitive point method).
For special apparatus by which, for instance, the .sup.31 P-nuclei or .sup.13 C-nuclei, of which relatively little occurs in the human body as compared to the .sup.1 H-nuclei, are excited or their nuclear spin resonance signals are received, RF coils are known which, for reasons of sufficient sensitivity, are arranged in regions of a body close to the surface, i.e., are placed in particular on the surface of the body. As RF coils suitable for this purpose, which therefore are also called surface coils, circular windings with a single or several turns are generally used. See the papers mentioned above.
The sensitivity maximum of such surface coils, however, is in regions close to the surface. If, however, parts of the body located deeper are specifically to be examined with these coils, in which their sensitivity is comparatively smaller, it cannot be avoided that the resonance signals to be obtained are always overlaid by relatively strong signals from the regions close to the surface. This makes the analysis of the signals more difficult, and often an unequivocal statement regarding the deeper parts of the body is impossible.